MP4032-2—OFFLINE LED DRIVER WITH FULLY-INTEGRATED INTERNAL MOSFET

MP4032-2 Rev.1.01

www.MonolithicPower.com

16

10/23/2014

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MULT Pin Resistor Divider

The MULT pin resistor divider requires careful

tuning because the MULT voltage determines the

COMP voltage level, which directly influences the

dimming curve and performance. Test the

estimated divider values with different types of

TRIAC dimmers to determine precise resistor

values: typically use the divider for a COMP level

of 2.3V at 120VAC input.

Add a ceramic X7R capacitor to the MULT pin to

absorb the switching-frequency ripple on the

MULT voltage for accurate dimming-phase

detection. Increasing the capacitance can further

smooth the MULT voltage, but increase the input-

line voltage phase shift and diminish the power

factor. Typical values are between 2.2nF and

8.2nF.

VCC Power Supply

After the system starts up, the auxiliary winding

takes over the VCC power supply through a

rectifying diode with a relatively small current-

limiting resistor because of the limited power

dissipation. The bulk capacitor stabilizes the VCC

voltage to limit the ripple—most applications use

22μF. Use the following equation to determine

the voltage rating of the rectifying diode:







aux

D

max

in _ max

aux _ negtive _ spike

L

N

VVCC

V

V

N

is the maximum negative spike on auxiliary

winding.

Layout Guide

PCB layout is very important to achieve reliable

operation,

and

good

EMI

and

thermal

performance. Follow these guidelines to optimize

performance.

1) Design a short main-power path. Directly

connect the sense resistor GND return to the

input capacitor. Use the largest-possible

copper pour for the power devices for good

thermal performance.

2) Separate the power GND and the analog

GND, and connect them together only at an

IC GND pin.

3) Place the components as close as possible to

the corresponding IC pins. The ZCD pin

bypass

capacitor

and

the

COMP

pin

capacitor have priority.

4) Separate the input high voltage wire from

other components and GND to avoid surge

failures. Select the pull-down resistors for the

BUS line to the active damping circuit and the

MULT pin for the DIP package.

5) Place the output rectifying diode as close as

possible to the output filter capacitor, and use

a short trace from the inductor output return

pin to the return point of the output filter

capacitor.